As is well known, nuclear materials can be smuggled into the United States by being hidden in cargo containers or vessels. Scanners have been developed that image cargo containers utilizing gamma rays. However, these scanners use low-intensity continuous gamma rays that are not powerful enough to pass through fully loaded cargo containers to detect nuclear materials hidden in the cargo.
Nuclear materials can be smuggled into the country amongst, for instance, frozen food, such as fish, in which the water within the food acts as a moderator to the radiation from any nuclear materials embedded in the food.
Moreover, nuclear materials can be hidden in lead containers such that passive techniques such as Geiger counters cannot be utilized to detect the presence of nuclear materials. Containers can be 40-foot trailers or smaller containers that go into aircraft, small ships or even fishing boats.
More particularly, in the past shipping containers that may be from 20 to 40 feet in length have been subjected to a passive screening system involving Geiger counter-type detection. In these systems, the Geiger counter is passed adjacent the containers looking for radioactive materials.
This system can be easily defeated because one can take the materials and put them inside a lead container such that radiation does not pass out of the container. One could also place the materials inside a large amount of water. One could therefore place the nuclear material adjacent a shipment of bottled water and utilize the bottled water as a shielding agent so that the nuclear material cannot be passively detected.
As a result, active screening approaches have been utilized that illuminate the cargo container, most usually made of steel, and to do so utilizing a technique that can see through the steel and see through most of the container contents. These types of systems include gamma ray sources, with the gamma rays being utilized for illumination.
Those systems that utilize gamma rays in general use continuous sources of gamma rays, with the sources not having a very high intensity. These sources thus are characterized by low brightness. The result of utilizing these sources is that the system cannot see through large amounts of cargo.
Thus if one has a container full of frozen fish, to the extent that the containers are full, this is equivalent to having a container full of water. As is well known, fish are 70% water and the gamma rays become absorbed so that they do not make it all the way through from one side of the container to the other.
These systems operate on the principle that one has a source on one side of the container and a detector on the other side of the container. If the container is filled with water or gamma-absorbing material, one does not get sufficient signal at the detector to be able to reliably ascertain what is in the container.
Similarly, if a container has cargo involving a large amount of metal, such as associated with moving machine parts, one needs to use very high-intensity gamma rays to penetrate from the source through the container to the detector.
It is noted that radioisotope sources are continuous sources because the gamma rays are emitted continuously, with the gamma intensity depending on the exponential decay of the radioisotope. These sources are not very bright unless one has a large amount of radioactive material. However, if one utilizes a large amount of radioisotope material, this presents its own threat. Moreover, if a terrorist wished to cause damage, he could, for instance, blow up the detector system itself to generate a cloud of radioactive material that could drift into populated areas.
Moreover, the use of large quantities of radioisotopes poses a threat to people who are working around the port or the docks, including customs officials that have to go by the radioactive source every day. Thus the radioactive source emits continuously and must be shielded and protected against terrorist attack.
These sources are usually contained in lead having an aperture that is controllable such that radiation comes out of the lead shielding only when the aperture is opened up to irradiate the container and then only in the direction associated with the aperture.
In practice one has a container placed on a truck bed, such that when the container is offloaded from the ship to the truck, it is then driven up to the radioactive detector. The truck stops and the driver gets out, after which the truck is moved through the scanner having this continuous radioactive source such that the container is passed by the source. The container passing by the source can also result in scattering off of the material in the container, again providing a health hazard.
Most importantly, with such continuous low-level sources to be effective at all, the scanning time for a 40-foot container is on the order of minutes. In a busy port facility, the speed at which the containers can be scanned in this manner limits the number of containers that can be scanned to something considerably less than 100% of all of the available containers.
There are other types of systems in which neutron particle streams penetrate material that is housed in the container, at which point one looks for fluorescence that is emitted as a result of bombardment with neutrons. For high-Z materials, having atomic weights equal to lead or higher, for instance uranium, one obtains characteristic signatures out from the irradiated material because as the neutrons come in and strike the nuclei, the nuclei relax and emit radiation in the form of fluorescence. However, this type of system is exceedingly slow in terms of scanning, mainly because of the low intensity of the neutron stream due to the small number of neutrons that can be obtained from the neutron source.
In most neutron-type scanning systems, the system utilizes a particle accelerator that accelerates particles that when broken apart produce neutrons.
One problem with such a system is that neutrons going through the contents of a container can damage the contents. In one sense, neutrons are more damaging than gamma rays.
Also the provision of a particle accelerator is impractical because of the large investment necessary for each scanning station. Most importantly, neutron beams present a safety hazard. This means that it is unsafe for personnel near the area. One therefore has to clear the area before the neutrons are emitted.
Moreover, the neutron source is again a low brightness source that requires a long scan time.
The systems that utilize a neutron beam sees high-Z material because as the neutrons hit the high-Z material, they get absorbed. This is true both for lead, and other high-Z materials such as uranium or plutonium.
Also the neutrons impinging upon various nuclei may cause fluorescence so that one obtains a signature characteristic of the embedded nuclear material, which is different from the signature that one might obtain from the cargo itself.
Note also, with respect to the neutron beam systems, one has to be able to accommodate various different types of containers and various different types of cargo, each of which can absorb the neutrons and cause the fluorescence. Thus the generation of a signature must take into account the various absorbing material between the source and the detector. Depending on the type of contents of the container, one might not be able to properly read the signature to determine the presence of high-Z material within the container.
In addition to the systems utilizing radiologic sources, there is yet another type of system that utilizes a LINAC, which is an accelerator to create gamma rays as opposed to using a radiologic source. LINAC-induced gamma rays are, however, of low intensity and likewise is a continuous source of gamma rays.
One therefore obtains a relatively low intensity over a number of seconds such that the average intensity that irradiates the cargo and comes out the other side is quite low. If one has low brightness in any given millisecond or nanosecond interval, then the detected result may be in the noise level.
Note that requirement is to be able to scan the container as quickly as possible and as safely as possible, with the accuracy in determining the presence of high-Z material being paramount.